One of the most popular and promising targets for HIV vaccine development are immunogens derived from gp120, an envelope glycoprotein essential for viral entry into CD4 cells. Recently, our collaborators have isolated multiple HIV broadly neutralizing antibodies to gp120 and shown that they require certain N-linked glycans for activity. However, some of these antibodies do not bind the free N-linked glycans, suggesting that at least some of them may also interact with the protein portion of gp120. The antibody.gp120 complexes too large for generation of high-resolution structures by NMR spectroscopy, and as the glycan appear to play key roles in the antibody epitopes, crystallization of the complexes is a daunting task. Computational modeling is an attractive approach for this problem, but purely computational approaches can generate models of questionable accuracy, and require empirical constraints or testing in order to generate a reliable model. We propose to characterize the various broadly neutralizing antibody epitopes using hydroxyl radical protein footprinting of the gp120-antibody complexes, a technique that labels a broad variety of amino acid side chains based on their accessibility to solvent. In order to improve the usefulness of the footprinting data for accurate, high-resolution model building, we propose to develop a number of improved footprinting methods, including accurate quantitation at the amino acid level to improve structural resolution and normalization protocols to generate absolute solvent accessible surface area values from footprinting data. We also propose to develop an appropriate scoring function to utilize solvent accessible surface areas as a constraint in molecular dynamics simulations, analogous to the use of distance constraints. From these improvements and their application to the characterization of gp120-antibody complexes, we anticipate the generation of accurate, experimentally-constrained models that correctly identify the epitope for each antibody. These models will be very important for the rational design of immunogens to raise the corresponding broadly neutralizing antibodies in a host through immunization.